1. Field of the Invention
The present invention relates to an image reading apparatus that performs shading correction, a method of controlling the image reading apparatus, and a storage medium.
2. Description of the Related Art
Generally, in an image reading apparatus that reads an original image using a reading sensor, such as a CCD and a CMOS, mismatching called shading is sometimes caused between an original luminance of an image and a read signal. More specifically, mismatching is caused by the influences of variation in reading characteristics of reading elements constituting the reading sensor, occurring on a pixel-by-pixel basis, unevenness of distribution of the amount of light in a main scanning direction, which is emitted from a light source for irradiating an original, distortion characteristics of a condensing lens that condenses light reflected from the original onto the reading sensor, and so on.
As a method of correcting mismatching in such a read image to thereby make the whole image uniform in brightness in plane on average, shading correction is generally performed. Shading correction generally employs a method in which a white reference member whose chromaticity is controlled is set within an image reading apparatus as an object from which target values of uniform brightness in shading correction are to be obtained, the white reference member is read before the start of reading an image of an original, and shading correction data is generated based on the reading result.
By the way, an image reading apparatus equipped with a first reading unit configured to read a front side of an original and a second reading unit configured to read a reverse side of the original is capable of reading both of the front and reverse sides of the original only by conveying the original once. In the image reading apparatus having such an original double-sided reading function, the second reading unit is usually provided in an original conveying section of the image reading apparatus, which makes it difficult to secure a space for disposing the white reference member. Therefore, in many cases, shading correction data is generated using a white reference document.
Conventionally, in the image reading apparatus equipped with the above-mentioned first reading unit and second reading unit, there has been proposed a method of creating second white reference data (see e.g. Japanese Patent Laid-Open Publication No. 2004-207790). In the apparatus described in Japanese Patent Laid-Open Publication No. 2004-207790, the second white reference data is created based on initial white reference data obtained by reading a second white reference member by the second reading unit and background data of an original obtained by reading the original by the second reading unit. By employing this method, it is possible to relax positional restrictions for providing the second white reference member within the image reading apparatus, and it is possible to simplify the apparatus construction.
Further, in an apparatus described in Japanese Patent Laid-Open Publication No. 2006-217481, whiteness correction data is generated based on a difference between data of respective pixels at the same position of initial second white reference member-read data and white reference document data, and second shading correction data is generated based on the generated whiteness correction data. Note that the initial second white reference member-read data is data obtained by reading a second white reference member which is disposed on an opposed surface to a second reading unit configured to read the second side of an original. Further, the white reference document data is data obtained by reading, using the second reading unit, a white reference document which is made of a material having whiteness at approximately the same level as that of a first white reference member disposed in the vicinity of a platen glass for a movable first reading unit.
Further, in an apparatus described in Japanese Patent Laid-Open Publication No. 2011-151478, first reference image data obtained by reading a white reference chart and second reference image data obtained by reading a white reference board are stored when the apparatus is shipped from the factory or when the apparatus is subjected to maintenance. Third reference image data obtained by reading the white reference board after the apparatus is powered on and before an original image is read is multiplied by the first reference image data, and is also divided by the second reference image data to thereby determine a correction value for shading correction. By employing this method, it becomes unnecessary to read the white reference chart whenever shading correction is performed, which makes it possible to perform accurate shading correction at desired timing.
However, in the apparatus described in Japanese Patent Laid-Open Publication No. 2004-207790, data used for shading correction may be largely varied depending on a condition of the background of an original. More specifically, in a case where an area on the original, including characters and a diagram, is read, data formed by reading portions including dust or stripes also forms part of the data for shading correction. Further, the data for shading correction is also varied in a case where a sheet having a background which is not white, such as colored paper, is used as an original, or a case where stripes are generated because dust, such as paper dust, remaining within an original conveying section of the image reading apparatus is attached to an original during conveyance of the original and is fixed at or floats around an original reading position. In these cases, it is impossible to acquire accurate shading correction data to be served as target values of uniform brightness which is the aim of shading correction.
Further, in the case of the apparatus described in Japanese Patent Laid-Open Publication No. 2006-217481, it is difficult to cause a position at which the white reference document is read by the first reading unit and a position at which reading is performed by the second reading unit to coincide with each other, when influences of original conveying accuracy and variation and aging of members constituting the apparatus are taken into account. Further, the first and second reading units are disposed at respective desired positions within the original conveying section of the image reading apparatus, and hence a condition of attachment of dust to each reading position is different, which makes different the position and the number of stripes generated from the dust.
Therefore, as shown in FIG. 17, in a case where the following stripes are generated when the white reference document is being conveyed, it is impossible to generate accurate data for shading correction. More specifically, in a case where the above-mentioned stripes caused by dust are generated on different positions on a front side of the original read by the first reading unit and a reverse side read by the second reading unit, it is impossible to generate accurate data for shading correction. FIG. 17 shows an example in which stripes caused by floating dust and a stripe caused by fixed dust are generated on the different positions on the front side image and the reverse side image, respectively.
Further, in the apparatus described in Japanese Patent Laid-Open Publication No. 2011-151478, if the first reference image data itself, based on which the shading correction data is calculated, is influenced by dirt of the white reference chart or stripes caused by dust generated during conveyance of the chart, there arises a problem that even if the shading correction data can be generated at a desired timing thereafter, accurate shading correction cannot be expected.
That is, even when any one of the methods disclosed in the above Japanese Patent Laid-Open Publication No. 2004-207790 and Japanese Patent Laid-Open Publication No. 2006-217481 is employed, unless data for shading correction is acquired in a state where strict management is executed with respect to dirt on the white reference chart to be used and dirt within the image reading apparatus, the effect of shading correction is largely reduced.